For more than a century paper, board and tissue have been manufactured by using a fiber furnish where the fibers have been dispersed in water, and the dilute furnish is introduced from a headbox onto a wire of a fiber web machine, where the water is removed through the wire and the fibers remain on the wire forming a web. Suggestions and attempts have been made during the last few decades to replace water as the fiber suspending medium with foam, as foam has some attractive properties, like for instance its capability of suspending solid particulates having greatly varying specific gravity. In other words, once solid particulates are mixed evenly into foam the foam bubbles carry the solid particulates, practically seen, in the same order on the wire of the fiber web machine. The foam bubbles maintain their position in the foam irrespective of the specific gravity or the weight of the solid particulate in the bubble.
Prior art knows a few methods of manufacturing paper or board by means of using foam instead of water as the fiber suspending medium.
U.S. Pat. No. 4,299,655 discusses a fibrous foam generator for papermaking machine. It discloses a dispersion distribution arrangement used in a paper making machine wherein foam is delivered through an upstream line by means of a fan pump to a dispersion distribution chamber. The distribution chamber is designed for the dispersion of fibers in foam, and for the regeneration of foam in its flow to a headbox. An upstream line introduces foam from a separate generating apparatus to the distribution chamber. A further input line may be provided with a discharge nozzle centrally located in the upstream line for the controlled input of air and in some instances, additional surfactant so that the additional foam regeneration may occur. The foam regeneration, and the distribution and dispersion of fibers occur within the chamber, which receives paper making stock from its preparation. The chamber is flat on its ends, and within the chamber is located a movable plug, which is provided with means for introducing the paper making stock to the chamber. The plug is arranged to be movable in the direction of the liquid flow. The plug leaves between itself and the internal walls of the chamber a narrow gap along which the foam flows and into which the paper making stock is introduced. The document teaches that the dispersion distribution chamber is positioned in front of the headbox so that the foam with the suspended fibers flows to a headbox and out through a slice opening onto a traveling forming surface, wherein the liquid passes through the forming surface and the distributed fibers form a mat on the traveling forming surface.
In accordance with the process discussed in U.S. Pat. No. 6,500,302B2 the foamed fiber furnish is made up from aqueous slurry of natural or synthetic fibers or mixtures of fibers and foamed liquid carrier just prior to its introduction to the headbox. The foam is withdrawn from foam storage silo to a positive displacement fan pump. A pulp slurry containing fibers is drawn from machine chest and is added to a foamed liquid comprising water, air and surfactant containing 55 to 80 percent air by volume for forming a foamed fiber furnish having a consistency in the range of from about 0.1 to about 3 weight percent fiber by means of simple mixing caused by natural turbulence and mixing inherent in the process elements at the inlet to the fan pump in the amount necessary to form the foamed-fiber furnish of the desired consistency.
In other words, in prior art foam processes relating to paper making the foam is generated in a separate apparatus, which is usually a powerful agitator, where surfactant is added to water and the mixture is agitated strongly to mix air therein. Normally the thus created foam contains 50-85% air by volume. As discussed above there are several optional devices for mixing fibers into the foam. Sometimes fibers are mixed in a so called foam pulper where foam and fibers are mixed by strong agitation before being pumped via the headbox of the fiber web machine onto the wire.
A clear problem in the above discussed foam processes is the complicated or at least expensive equipment needed for the production and regeneration of foam. The same equipment also occupies a considerable space at the mill and has a high energy consumption. Sometimes, especially in cases where existing paper mills using conventional water-based furnish are planned to be converted to utilising foam formation, the volumes the foam requires are so high that there is no room for such equipment that would be able to maintain the earlier production capacity but a part of the capacity has to be sacrificed. In prior art processes the formation of the foam itself has been performed, for natural reasons, relatively far from the headbox, which means that the foam has to be transferred from its production to the headbox via a pipeline, whereby both the design and use of the pipeline is a challenging task. A further problem relates to the pumping of foam, as the foam cannot be pumped with off-the-shelf standard centrifugal pumps, but specifically designed pumps have to be used. This is yet another factor increasing the additional costs involved in the foam process compared to traditional paper making.